Heart failure (HF) is a medical condition that occurs when the heart is unable to pump sufficiently to sustain the organs of the body. Heart failure is a serious condition and affects millions of patients in the United States and around the world.
In the United States alone, about 5.1 million people suffer from heart failure and according to the Center for Disease Control, the condition costs the nation over $30 billion in care, treatments, medications, and lost production. The normal healthy heart is a muscular pump that is, on average, slightly larger than a fist. It pumps blood continuously through the circulatory system to supply the body with oxygenated blood. Under conditions of heart failure, the weakened heart cannot supply the body with enough blood and results in cardiomyopathy (heart muscle disease) characterized by fatigue and shortness of breath, making even everyday activities such as walking very difficult.
Oftentimes, in an attempt compensate for this dysfunction, the heart and body undergo physiological changes that temporarily mask the inability of the heart to sustain the body. These changes include the enlargement of heart chamber, increased cardiac musculature, increased heart rate, raised blood pressure, poor blood flow, and imbalance of body fluids in the limbs and lungs.
One common measure of heart health is left ventricular ejection fraction (LVEF) or ejection fraction. By definition, the volume of blood within a ventricle immediately before a contraction is known as the end-diastolic volume (EDV). Likewise, the volume of blood left in a ventricle at the end of contraction is end-systolic volume (ESV). The difference between EDV and ESV is stroke volume (SV). SV describes the volume of blood ejected from the right and left ventricles with each heartbeat. Ejection fraction (EF) is the fraction of the EDV that is ejected with each beat; that is, it is SV divided by EDV. Cardiac output (CO) is defined as the volume of blood pumped per minute by each ventricle of the heart. CO is equal to SV times the heart rate (HR).
Cardiomyopathy, in which the heart muscle becomes weakened, stretched, or exhibits other structural problems, can be further categorized into systolic and diastolic dysfunction based on ventricular ejection fraction.
Systolic dysfunction is characterized by a decrease in myocardial contractility. A reduction in the LVEF results when myocardial contractility is decreased throughout the left ventricle. CO is maintained in two ways: left ventricular enlargement results in a higher SV and an increase in contractility as a result of the increased mechanical advantage from stretching the heart. However, these compensatory mechanisms are eventually exceeded by continued weakening of the heart and CO decreases, resulting in the physiologic manifestations of HF. The left side of the heart cannot pump with enough force to push a sufficient amount of blood into the systemic circulation. This leads to fluid backing up into the lungs and pulmonary congestion. In general terms, systolic dysfunction is defined as an LVEF less than 40% and heart failure in these patients can be broadly categorized as heart failure with reduced ejection fraction (HFrEF).
Diastolic dysfunction refers to cardiac dysfunction in which left ventricular filling is abnormal and is accompanied by elevated filling pressures. In diastole, while the heart muscle is relaxed the filling of the left ventricle is a passive process that depends on the compliance (defined by volume changes over pressure changes), or distensibility, of the myocardium or heart muscle. When the ventricles are unable to relax and fill, the myocardium may strengthen in an effort to compensate to poor SV. This subsequent muscle hypertrophy leads to even further inadequate filling. Diastolic dysfunction may lead to edema or fluid accumulation, especially in the feet, ankles, and legs. Furthermore, some patients may also have pulmonary congestion as result of fluid buildup in the lungs. For patients with HF but without systolic dysfunction, diastolic dysfunction is the presumed cause. Diastolic dysfunction is characteristic of not only HCM, which is characterized by the thickening of heart muscle, but also RCM, which is characterized by rigid heart muscle that cannot stretch to accommodate passive filling. In general terms, diastolic dysfunction is defined as a LVEF of greater than 40% and HF in these patients can be broadly categorized as heart failure with preserved ejection fraction (HFpEF).
While a number of drug therapies successfully target systolic dysfunction and HFrEF, for the large group of patients with diastolic dysfunction and HFpEF no promising therapies have yet been identified. The clinical course for patients with both HFrEF and HFpEF is significant for recurrent presentations of acute decompensated heart failure (ADHF) with symptoms of dyspnea, decreased exercise capacity, peripheral edema etc. Recurrent admissions for ADHF utilize a large part of current health care resources and could continue to generate enormous costs.
While the pathophysiology of HF is becoming increasingly better understood, modern medicine has, thus far, failed to develop new therapies for chronic management of HF or recurrent ADHF episodes. Over the past few decades, strategies of ADHF management and prevention have and continue to focus on the classical paradigm that salt and fluid retention is the cause of intravascular fluid expansion and cardiac decompensation. Increasing evidence suggests that fluid homeostasis and control of intravascular fluid distribution is disrupted in patients with HF. Disregulation of this key cardiovascular regulatory component could not only explain findings in chronic HF but also in ADHF. Consequently, blocking of the autonomic nervous system to alter fluid distribution in the human body could be used as a therapeutic intervention.
Additionally, the classical understanding of HF pathophysiology emphasizes the mechanism of poor forward flow (i.e., low CO), resulting in neurohumoral, or sympathetic nervous system (SNS) up-regulation. However, new evidence emphasizes the concurrent role of backward failure (i.e., systemic congestion) in the pathophysiology and disease progression of HF. Coexisting renal dysfunction with diuretic resistance often complicates the treatment of HF and occurs more frequently in patients with increased cardiac filling pressures. Chronic congestive heart failure (CHF) is characterized by longstanding venous congestion and increased neurohumoral activation. Critically important has been the identification of the splanchnic vascular bed as a major contributor to blood pooling and cardiac physiology. Newly evolving methods and devices involving sympathetic nervous system blocking and manipulation of systems including the splanchnic vascular bed have opened novel avenues to approach the treatment of heart disease. In particular, the role of sympathetic nerves that innervate smooth muscle in the walls of splanchnic veins have become better known. In the case of hyperactivity of these nerves they became a novel target in the treatment of CHF.